1nbdcopy(1) LIBNBD nbdcopy(1)
2
6 nbdcopy - copy to and from an NBD server
7
9 nbdcopy [--allocated] [-C N|--connections=N]
10 [--destination-is-zero|--target-is-zero] [--flush]
11 [--no-extents] [-p|--progress|--progress=FD]
12 [--request-size=N] [-R N|--requests=N] [-S N|--sparse=N]
13 [--synchronous] [-T N|--threads=N] [-v|--verbose]
14 SOURCE DESTINATION
15 SOURCE, DESTINATION := - | FILE | DEVICE | NBD-URI | [ CMD ARGS ... ]
17 DESTINATION += null:
18 nbdcopy --help
20 nbdcopy -V|--version
22
24 nbdcopy nbd://example.com local.img
25 This copies everything from the NBD server at "example.com" to a local
26 file called local.img.
27 nbdcopy nbd://example.com - | file -
29 This streams the first part of the disk on the NBD server at
30 "example.com" into the file(1) command. Note here that "-" means to
31 stream to stdout (and therefore into the pipe to the file command).
32 nbdinfo(1) is another way to detect the content on an NBD server.
34 nbdcopy -p /dev/sdX "nbd+unix:///?socket=/tmp/unixsock"
36 Copy the full local hard disk "/dev/sdX" to the NBD server listening on
37 the Unix domain socket /tmp/unixsock. Because of the -p option this
38 will print a progress bar.
39 nbdcopy nbd://server1 nbd://server2
41 Copy a full disk from one NBD server to another.
42 nbdcopy -- [ qemu-nbd -r -f qcow2 https://example.com/disk.qcow2 ] -
44 Run qemu-nbd(8) as a subprocess to open URL
45 "https://example.com/disk.qcow2", which is then streamed to stdout
46 ("-"), converted to blocks of raw format data. Note "--" to prevent
47 qemu-nbd flags from being interpreted as nbdcopy flags.
48 cat disk1 disk2 | nbdcopy -- - [ qemu-nbd -f qcow2 output.qcow2 ]
50 Concatenate two raw-format disk images into the qcow2 file
51 output.qcow2. The output file has to be precreated.
52 nbdcopy nbd://server null:
54 Read the contents of the NBD server as fast as possible and throw it
55 away (write it to a virtual null device). This is useful for
56 benchmarking NBD servers and/or nbdcopy.
57
59 nbdcopy copies to and from an NBD server. It can upload a local file
60 to an NBD server, or download the contents of an NBD server to a local
61 file, device or stdin/stdout. It can also copy between NBD servers.
62 The local file can be a file, a block device (eg. "/dev/cdrom"), or "-"
64 which means stream in from stdin or stream out to stdout.
65 The NBD server can be specified using an NBD URI (like
67 "nbd://localhost"). The NBD server can be local or remote, and
68 encryption can be used if libnbd was built with encryption support.
69 Alternately you can use square brackets around a qemu-nbd(8) or
70 nbdkit(1) command to run the NBD server as a subprocess of nbdcopy.
71 The destination may be the special word "null:" to throw away the
73 output.
74 For more complex copying operations including converting between disk
76 formats use "qemu-img convert", see qemu-img(1).
77
79 --help
80 Display brief command line help and exit.
81 --allocated
83 Normally nbdcopy tries to create sparse output (with holes) if the
84 destination supports that. It does this in two ways: either using
85 extent information from the source to copy holes (see
86 --no-extents), or by detecting runs of zeroes (see -S). If you use
87 --allocated then nbdcopy creates a fully allocated, non-sparse
88 output on the destination.
89 -C N
91 --connections=N
92 Set the maximum number of NBD connections ("multi-conn"). By
93 default nbdcopy will try to use multi-conn with up to 4 connections
94 if the NBD server supports it. If copying between NBD servers then
95 nbdcopy cannot use multi-conn if either of the servers does not
96 support it.
97 --destination-is-zero
99 --target-is-zero
100 Assume the destination is already zeroed. This allows nbdcopy to
101 skip copying blocks of zeroes from the source to the destination.
102 This is not safe unless the destination device is already zeroed.
103 (--target-is-zero is provided for compatibility with qemu-img(1).)
104 --flush
106 Flush writes to ensure that everything is written to persistent
107 storage before nbdcopy exits.
108 --no-extents
110 Normally nbdcopy uses extent metadata to skip over parts of the
111 source disk which contain holes. If you use this flag, nbdcopy
112 ignores extent information and reads everything, which is usually
113 slower. You might use this flag in two situations: the source NBD
114 server has incorrect metadata information; or the source has very
115 slow extent querying so it's faster to simply read all of the data.
116 -p
118 --progress
119 Display a progress bar.
120 --progress=FD
122 Write a progress bar to the file descriptor "FD" (a number) in a
123 format which is easily parsable by other programs. nbdcopy will
124 periodically write the string "N/100\n" (where N is an integer
125 between 0 and 100) to the file descriptor.
126 To get nbdcopy to write the progress bar to a file you can use the
128 following shell commands:
129 exec 3>/tmp/progress
131 nbdcopy --progress=3 ...
132 exec 3>&-
133 --request-size=N
135 Set the maximum request size in bytes. The maximum value is 32 MiB,
136 specified by the NBD protocol.
137 -R N
139 --requests=N
140 Set the maximum number of requests in flight per NBD connection.
141 -S N
143 --sparse=N
144 Detect all zero blocks of size N (bytes) and make them sparse on
145 the output. You can also turn off sparse detection using -S 0.
146 The default is 4096 bytes.
147 --synchronous
149 Force synchronous copying using the libnbd(3) synchronous ("high
150 level") API. This is slow but may be necessary for some broken NBD
151 servers which cannot handle multiple requests in flight. This mode
152 is also used when streaming to and from stdio, pipes and sockets.
153 -T N
155 --threads=N
156 Use up to N threads for copying. By default this is set to the
157 number of processor cores available.
158 Note --threads=0 means autodetect and --threads=1 means use a
160 single thread.
161 -v
163 --verbose
164 Verbose output. This enables debug in libnbd (see
165 nbd_set_debug(3)) as well as printing other useful information.
166 -V
168 --version
169 Display the package name and version and exit.
170
172 The three options --connections, --threads and --requests are related
173 and control the amount of parallelism available. The defaults should
174 ensure a reasonable amount of parallelism if possible and you don’t
175 need to adjust them, but this section tries to describe what is going
176 on.
177 Firstly if either side of the copy is streaming to or from stdio, a
179 pipe, or a socket, or if you use the --synchronous option, then nbdcopy
180 works in synchronous mode with no parallelism, and nothing else in this
181 section applies.
182 The --connections=N option controls NBD multi-conn (see "Multi-conn" in
184 libnbd(3)), opening up to N connections to the NBD server (or to both
185 NBD servers if copying between NBD servers). This defaults to 4. The
186 NBD servers must support and advertise multi-conn. For nbdkit(1)
187 availability of multi-conn can depend on the plugin. You can use
188 nbdinfo(1) to find out if a particular NBD server is advertising multi-
189 conn. If the NBD server doesn’t advertise multi-conn then only one
190 connection will be opened regardless of the --connections flag.
191 When copying between two NBD servers, the number of connections is
193 limited to the minimum multi-conn supported on both sides. For the
194 purposes of this calculation, you can consider local files and block
195 devices as supporting infinite multi-conn.
196 When you run an NBD server as a subprocess (using the "[ ... ]" syntax)
198 multi-conn cannot be used.
199 The --threads=N option allows nbdcopy to start up to N threads
201 (defaulting to the number of cores). However nbdcopy cannot use more
202 threads than the number of NBD connections.
203 The --requests=N option controls the maximum number of requests in
205 flight on each NBD connection. This enables the NBD server to process
206 requests in parallel even when multi-conn isn’t available or when using
207 a single thread. The default is chosen to allow a reasonable amount of
208 parallelism without using too much memory.
209 Because of this parallelism, nbdcopy does not read or write blocks in
211 order. If for some reason you require that blocks are copied in strict
212 order then you must use --synchronous.
213
215 Instead of connecting to an already running server using an NBD URI,
216 you can run an NBD server as a subprocess using:
217 nbdcopy -- [ CMD ARGS ... ] ...
219 This requires the server to support systemd socket activation, which
221 both qemu-nbd(8) and nbdkit(1) support (see also
222 nbd_connect_systemd_socket_activation(3)).
223 "[" and "]" must be separate command line parameters. You will usually
225 need to use "--" to stop nbdcopy from misinterpreting NBD server flags
226 as nbdcopy flags. Both the source and destination may be subprocesses.
227 nbdcopy cleans up the subprocess on exit.
228 Some examples follow.
230 nbdcopy -- [ qemu-nbd -f qcow2 disk.qcow2 ] - | hexdump -C
232 In this example, qemu-nbd(8) is run as a subprocess. The subprocess
233 opens disk.qcow2 and exposes it as NBD to nbdcopy. nbdcopy streams
234 this to stdout ("-") into the pipe which is read by hexdump(1).
235 nbdcopy -- [ qemu-nbd -f qcow2 disk.qcow2 ] [ nbdkit memory 1G ]
237 Two subprocesses are created, qemu-nbd(8) as the source and nbdkit(1)
238 as the destination. The qcow2 file is converted to raw and stored
239 temporarily in the RAM disk (nbdkit-memory-plugin(1)).
240 When nbdcopy exits both servers are killed and the RAM disk goes away,
242 so this command has no overall effect, but is useful for testing.
243
245 You can use nbdcopy, cmp(1) and bash(1) process substitution to compare
246 the content of two NBD servers for equality:
247 cmp <( nbdcopy nbd://server1 - ) <( nbdcopy nbd://server2 - )
249 Note this tests that the content is logically equal. It does not
251 compare the NBD metadata such as sparseness (see nbdinfo(1) --map
252 option). Thus for example a run of allocated zeroes in one server will
253 match a hole in the other.
254
256 libnbd(3), nbdfuse(1), nbdinfo(1), nbdsh(1), nbdkit(1), qemu-img(1).
257
259 Richard W.M. Jones
260
262 Copyright (C) 2020-2021 Red Hat Inc.
263
265 This library is free software; you can redistribute it and/or modify it
266 under the terms of the GNU Lesser General Public License as published
267 by the Free Software Foundation; either version 2 of the License, or
268 (at your option) any later version.
269 This library is distributed in the hope that it will be useful, but
271 WITHOUT ANY WARRANTY; without even the implied warranty of
272 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
273 Lesser General Public License for more details.
274 You should have received a copy of the GNU Lesser General Public
276 License along with this library; if not, write to the Free Software
277 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
278 02110-1301 USA
279libnbd-1.7.12 2021-05-29 nbdcopy(1)